Experiments were initiated with two mouse cell lines, a parental line carrying at least one integrated copy of SV40 DNA per diploid equivalent of cell DNA and a subline obtained by immunoselection carrying about 0.5 copy of SV40 DNA per diploid equivalent of cell DNA. Cloned derivatives of both lines were stable. The parental line synthesized early SV40 RNA and expressed early viral antigens. The subline produced no virus-specific RNA or antigens. Electropherograms of restriction endonuclease digests of the cellular DNAs, denatured and transferred to nitrocellulose paper, were hybridized with labeled SV40 DNA to localize SV40, demonstrated two loci of integrated SV40 DNA in each cell line. Hpa I digests of the parental cell line contained 4 fragments of SV40 DNA in common with the subline, but also contained 5 fragments of SV40 DNA absent from the subline. Other restriction enzyme digests supported the conclusion that a complex series of deletions from both loci of integrated SV40 DNA in the parental line were represented in the subline and further study was discontinued.